v1.3.0

• Enhanced documentation for ReactPHP's new HTTP client and
  add support / sponsorship info.
  (#21 and #24 by @clue)

• Improve test suite and add .gitattributes to exclude dev files from exports.
  Prepare PHP 8 support, update to PHPUnit 9 and simplify test matrix.
  (#22, #23 and #25 by @SimonFrings)